A typical solid-state image sensor comprises a light gathering stage and a readout chain. An example of such a sensor is described in S. K. Mendis, S. E. Kemeny, R. C. Gee, B. Pain, C. O. Staller, Q. Kim, E. R. Fossum, “CMOS active pixel sensors for highly integrated imaging systems”, IEEE J. Solid-State Circuits, vol. 32, pp. 187-197, Feb. 1997. The light gathering stage comprises a photodiode or other type of light sensitive element and a charge to voltage conversion means, while the readout chain typically comprises a source follower (present for all active pixels), a sampling circuit, a preamplifier, and an analog to digital conversion circuit.
Each of these components introduces noise to the detected image. The light contains photon shot noise, the photodiode introduces dark current shot noise, and fixed pattern noise, and the charge to voltage conversion means introduces transfer gate noise (pinned-photodiode pixels) or reset noise (3T active pixels).
The readout chain attempts to convert the detected voltage to digital form with the minimum amount of additional noise. However, each component of the readout chain introduces noise to the process. The pixel source follower introduces both thermal and 1/f noise. The-pixel transistors must be small to achieve a small pixel size and high fill factor and this compromises the noise level achievable. This noise source is often dominant over the remaining circuit noise.
The sampling circuit is used to perform a double sampling step, using capacitors contained in the base of each pixel column. Two samples are taken and differenced to eliminate pixel source follower threshold (Vt) mismatch, which gives two contributions of kT/C noise. To achieve small chip sizes and fast readout times the size of the sampling capacitors is limited which therefore limits the minimum achievable noise level.
Next, the preamplifier introduces thermal and 1/f noise. Often some means of removing amplifier column offsets is applied (auto-zero or feedback) which tends to increase this noise. Finally, the analog to digital conversion circuit comprises a quantization element, or quantizer, that introduces additional thermal noise, and a ramp generator that contributes quantization noise and thermal noise. The quantization element is required to convert from analog to digital output levels and compare the input voltage levels with a ramp voltage.
When designing an image sensor circuit, it is also desirable to minimize the surface area on the image sensor substrate that the circuit occupies. There is therefore often a trade-off between noise reduction and size considerations, for example, when considering the size (or capacitance) of capacitors to be used. Excessive noise degrades the performance of an image sensor, while an excessively large circuit will limit the applications to which the image sensor can be put or be commercially unattractive. It is therefore desirable to have an image sensor that contributes less noise and that also makes efficient use of space.